1. Field of the Invention
The present invention relates to the fermentation industry, and more specifically relates to a method for producing L-glutamic acid using a coryneform bacterium. L-Glutamic acid is widely used as a raw material in seasonings.
2. Brief Description of the Related Art
L-glutamic acid is conventionally produced by fermentation using coryneform bacteria belonging to the genus Brevibacterium, Corynebacterium or the like, which are able to produce L-glutamic acid. In order to improve productivity of these coryneform bacteria, strains isolated from nature, mutant strains, or strains modified by gene recombination are typically used.
Wild-type strains of coryneform bacteria do not produce L-glutamic acid when excess biotin is present in the fermentation medium. Therefore, conventional L-glutamic acid production methods restrict the amount of biotin by adding a surfactant or lactam antibiotic, which are known biotin suppressing agents, at the beginning or throughout the culture. However, many inexpensive raw materials used as carbon sources in culturing contain an excessive amount of biotin, such as blackstrap molasses. In such a case, the necessary addition of the above-mentioned biotin suppressing agent can greatly increase the cost of production (Biosci. Biotech. Biochem., 61(7), 1109-1112, 1997).
The dtsR gene, which encodes the DTSR protein from a Corynebacterium, was identified and found to impart resistance to a surfactant to the bacteria. Furthermore, when this gene is disrupted in an L-glutamic acid producing coryneform bacterium, a marked amount of L-glutamic acid is produced, even when biotin is present in an amount that would be significantly inhibitory in wild-type strains. Also, when the dtsR gene is amplified in an L-glutamic acid producing coryneform bacterium, the ability of the bacterium to produce L-glutamic acid is enhanced.
Moreover, it has also been found that if temperature sensitivity to biotin suppressing agents is imparted to an L-glutamic acid producing coryneform bacterium, the bacterium can stably produce L-glutamic acid by fermentation even in the presence of biotin. Furthermore, if the ability to produce L-lysine is imparted to this stain, L-lysine and L-glutamic acid can be simultaneously produced by fermentation even in the presence of biotin. It has also been found that using a variant dtsR gene produced by gene substitution which encodes a temperature sensitive DTSR protein imparts temperature sensitivity to a biotin suppressing agent to coryneform bacteria (International Patent Publication WO96/06180).
However, a gene which improves sensitivity to a surfactant when overexpressed in coryneform bacteria has not been reported.
Non-patent document 2: Biochem. Biophys. Res. Commun., 1997 May 8, 234(1):157
Patent document 1: International Patent Publication WO95/23224